When reading a book, the eyes of the reader are focused on the plane of the page, i.e. the image plane. The focus of a reader can deviate small amounts from exact focus on the image plane; however, larger deviations result in a loss of focus on the image plane.
In terms of reading, the loss of focus frequently results in loss of information. In particular, when a reader's eyes are focused on an image plane, visual information in plane is readily "transducible", the image falling on the retina being well defined. Accordingly, the retinal cells are optimally stimulated and the symbolic structure of the visual stimulus is optimally transmitted to the brain. However, the visual stimulus of an image perceived and momentarily stored by the retinal cells rapidly decays under conditions of incomplete "transduction". Such incomplete transduction occurs when the reader's eyes deviate from a particular image plane, causing a loss of focus in that plane.
Such loss of focus occurs whenever one of a reader's eyes drifts or changes focus, the other eye automatically adjusting or changing its focus as well. This is called "compensation" and it affects the binocular focal length of the eyes. The binocular focal length of the eyes is the distance between the plane of the eyes and the plane on which both eyes are focused. The amount of compensation is a function of both the amount of angular drift of a particular eye and the time over which that drift occurs. Usually, a larger drift occurs over a longer period of time, resulting in a greater compensatory change in the binocular focal length of the eyes. If the angular drift is sufficiently great, there is a complete loss of focus in the original image plane, resulting in a loss of stimulus data stored in the retinal cells.
If the above phenomena occurs when reading a book, the reader must re-focus on the image plane of the page. Upon restoring focus to the plane of the page, the reader frequently has trouble remembering where he was because the stimulus providing for such was lost with the loss of focus. This phenomenon can be very frustrating to readers and particularly to young persons just learning to read.
For example, if the left eye of the reader travels some angular distance to its left due to slack in the six pairs of extrensic eye muscles that control its horizontal movement, the right eye immediately compensates for the change in focal length by moving its position to the right. Focus is restored, but on a plane different from that of the original image plane, i.e., that of the plane of the page being read. The information on the image plane is not available, it being out of focus. Accordingly, that portion of the information from the image plane which had not been received and transmitted by the optic nerve to the brain is lost. Accordingly, after refocusing on the original image plane, the reader must search for his last-remembered reference point on the original image plane. This phenomenon is commonly termed "losing one's place" while reading.
Young children frequently utilize visual aids in maintaining their binocular focus on the plane of a page being read. For example, a child frequently uses his finger to maintain his position on a page.